A flip chip is generally a monolithic surface mount (SM) semiconductor device, such as an integrated circuit, having bead-like terminals formed on one of its surfaces. The terminals, typically in the form of solder bumps, serve to both secure the chip to a circuit board and electrically interconnect the flip chip circuitry to a conductor pattern formed on the circuit board, which may be a ceramic substrate, printed wiring board, flexible circuit, or a silicon substrate. Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of solder bumps are required. The solder bumps are typically located at the perimeter of the flip chip on electrically conductive pads that are electrically interconnected with the circuitry on the flip chip. The size of a typical flip chip is generally on the order of a few millimeters per side, resulting in the solder bumps being crowded along the perimeter of the flip chip. As a result, flip chip conductor patterns are typically composed of numerous individual conductors that are typically spaced apart about 0.5 millimeter or less.
Because of the narrow spacing required for the solder bumps and conductors, soldering a flip chip to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder on the flip chip using methods such as electrodeposition. Once deposited, heating the solder above its liquidus temperature serves to form the characteristic solder bumps on the surface of the flip chip. After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reheating, or reflowing, the solder so as to metallurgically adhere, and thereby electrically interconnect, each solder bump with its corresponding conductor, forming what will be referred to herein as a solder bump connection.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the height of the solder bump connections after soldering. As is well known in the art, controlling the height of solder bump connections after reflow is often necessary to prevent the surface tension of the molten solder bumps from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between the chip and its substrate, which may be termed the "stand-off height," is desirable for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable processing residues, and enabling the penetration of mechanical bonding and encapsulation materials between the chip and its substrate.
Solder bump position and height are generally controlled by the amount of solder deposited on the flip chip to form the solder bump and/or by limiting the surface area over which the solder bump is allowed to reflow. As illustrated in FIG. 1 showing a conductor 12 shown in longitudinal cross-section, the latter approach typically involves the use of solder stops 14, which can be formed by a solder mask or printed dielectric. The solder stops 14 are shown as extending widthwise across the surface 18 of the conductor 12, which has been printed or otherwise formed on a circuit substrate 10. A flip chip solder bump 16 (minus the flip chip) is shown as being positioned at the surface 18 of the conductor 12, as would be the case after a flip chip has been registered but before being reflow soldered to the conductor 12. As is apparent from FIG. 1, the solder stops 14 delineate a rectangular-shaped area on the surface 18 of the conductor 12 over which the solder bump 16 is able to flow during reflow. By properly locating the solder stops 14 on the conductor 12, the degree to which the molten solder can spread during reflow is controlled, which in turn determines the height of the solder bump 16 and therefore the stand-off height of the flip chip relative to the substrate 10. Within certain limits, a smaller reflow area results in a greater solder bump height after reflow for a given quantity of solder, though an excessively small reflow area tends to cause the solder bump to yield a poor connection due to inadequate adhesion.
Because the flip chip solder bump 16 is registered and soldered directly to the conductor 12, the conductor 12 must be formed of a solderable material, which as used herein means that a tin, lead or indium-based alloy is able to adhere to the conductor 12 through the formation of a metallurgical bond. In contrast, the solder stops 14 are intentionally formed of a nonsolderable material, meaning that a tin, lead or indium-based solder will not adhere to the material for failure to form a metallurgical bond. Upon reflow, the rectangular-shaped reflow area formed by the solder stops 14 on the conductor 12 yields a solder bump connection having a semi-spherical shape.
While flip chip conductors equipped with solder stops as shown in FIG. 1 are widely used in the art, trends in the industry have complicated the ability for solder bumps with solder stops to yield solder connections that provide adequate stand-off heights for flip chips. Specifically, as flip chips have become more complex, the number of bumps that must be accommodated along the chip perimeter has increased. In turn, the conductors to which the bumps are registered and soldered have become more closely spaced and narrower, e.g., line widths of about 0.4 millimeter or less. Such circumstances have complicated the design and fabrication of solder stops. As a result, solder bump connections having adequate stand-off height are more difficult to consistently produce, which increases the difficulty of adequately dispersing encapsulation materials between flip chips and their substrates. Inadequate stand-off height also corresponds to reduced compliance of the solder connection, rendering the solder connections more susceptible to fatigue fracture, and increases the difficulty with which solder flux residue is removed following the solder process. Finally, if inadequate stand-off height occurs with fine pitch conductors, interaction and contact between adjacent solder bump connections are also more likely to occur due to excessive lateral flow of the solder during reflow.
Accordingly, it would be desirable if an improved method were available for controlling the stand-off height of a surface mount device following reflow in which a solder bump connection is formed to mechanically mount the device to a surface. It would also be desirable if such a method was able to control the shape of a solder bump connection by carefully shaping the solderable area on a conductor, yet eliminated the use of solder stops. Finally, it would be desirable if such a method was particularly applicable to surface mount devices having fine pitch terminal patterns.